1. Field of the Invention
The present invention relates to a gas engine having a gas fuel reforming device that reforms a gas fuel such as natural gas with a thermal energy of exhaust gas to enhance the thermal efficiency.
2. Description of the Prior Art
Gas engines using natural gas as a main fuel are being developed as cogeneration systems. The cogeneration system extracts power in the form of electric energy by a generator and also heats water with the heat of the exhaust gas by a heat exchanger to produce hot water for use in a hot-water-supply system.
Engines using natural gas as a fuel includes, for example, Japanese Patent Laid-Open No. 108865/1994 and 101495/1994.
The cogeneration type gas engine disclosed in Japanese Patent Laid-Open No. 108865/1994 passes exhaust gas through a turbocharger, an energy recovery device and a steam generation device to reduce the exhaust gas temperature and then uses the reduced temperature exhaust gas in an exhaust gas recirculation (EGR) system to reduce NO.sub.x emissions. It also drives the turbocharger with the exhaust gas from a heat insulating type gas engine and, by using exhaust gas from the turbocharger, drives the energy recovery device having a generator. The cogeneration type gas engine sends the exhaust gas from the energy recovery device to the steam generation device of the heat exchanger where water is converted into steam, which is then used to drive a steam turbine to recover electric energy.
The multicylinder gas engine disclosed in Japanese Patent Laid-Open No. 101495/1994 provides a turbocharger to each group of two cylinders that are not consecutive in the ignition order, with first and fourth cylinders connected to one of exhaust manifolds and second and third cylinders connected to the other manifold. The both exhaust manifolds are provided with a water nozzle from which to spout water. The water spouted from the nozzles into the exhaust manifolds is vaporized in the exhaust passage and converted into steam, increasing the gas flow and driving the turbocharger.
In the gas engine when the combustion chambers are made of ceramics and constructed in a heat insulating structure, the air compression temperature increases to more than a self-ignition temperature of the natural gas, obviating an ignition device. Further, precombustion chambers, in which fuel is introduced, are provided in the gas engine in addition to the main combustion chambers in which air is introduced, and a control valve is installed between the main combustion chamber and the precombustion chamber to offer a diesel-cycle-based cogeneration engine with high efficiency. The exhaust gas of the gas engine, when the combustion chamber is constructed in a heat insulating structure, reaches as high a temperature as 850.degree. C. It is thus possible to recover thermal energy from the high-temperature exhaust gas to improve the heat efficiency of the engine.
The natural gas is known to have methane CH.sub.4 as its major component. Methane as a fuel has a high calorific value and occurs abundantly in the natural environment and thus expectations are growing for methane to become a future replacement fuel for oil. Methane, when reformed by thermal decomposition through a catalyst, is transformed into carbon monoxide CO and hydrogen H.sub.2 and the calorific values of CO and H.sub.2 are greater than that of CH.sub.4. When used in engines, methane improves the thermal efficiency, contributing to the conservation of resources and the reduction of CO.sub.2 emissions.
The calorific values of methane CH.sub.4, carbon monoxide CO and hydrogen H.sub.2 as fuels are as follows: EQU CH.sub.4 +2O.sub.2 .fwdarw.CO.sub.2 +2H.sub.2 O+212,800 kcal/kmol.multidot. W EQU CO+(1/2)O.sub.2 .fwdarw.CO.sub.2 +67,700 kcal/kmol.multidot.W EQU H.sub.2 +(1/2)O.sub.2 .fwdarw.H.sub.2 O+68,350 kcal/kmol.multidot.W
When on the other hand CH.sub.4 is reformed by using a catalyst and the thermal energy of exhaust has, the calorific value of the fuel increases. EQU CH.sub.4 +CO.sub.2 .fwdarw.2CO+2H.sub.2 EQU 2CO+2H.sub.2 +2O.sub.2 EQU .fwdarw.2CO.sub.2 +2H.sub.2 O+272,100 kcal/kmol.multidot.W